A receiver for use in a telecommunications systems typically comprises a number of different elements, such as a low noise amplifier (LNA), a mixer and a baseband filter, each of which has variable gain and selectivity. In receivers of this type an automatic gain control (AGC) is typically used to set the gains of different elements of the receiver, such that none of the components saturates when a signal is received, whilst also ensuring that the quality of the signal at an output of the receiver is high enough to permit demodulation/decoding of the transmitted data contained in the received signal.
In some receivers, such as those operating in accordance with the IEEE802.11 (WiFi) standard, AGC hardware operates continuously, but is frozen shortly after the beginning of a data frame is received by the receiver. After this freeze of the AGC hardware, the gains of the components of the receiver can no longer be changed. However, interference may commence after the freeze of the AGC hardware, and this interference can cause saturation of elements of the receiver. This can lead to a situation in which the beginning of a data frame is correctly received (i.e. the transmitted data contained in the received data frame can be correctly demodulated/decoded), but the end of the data frame is not.
In such situations it is possible that setting the gains of the elements of the receiver to lower values would have permitted successful reception of the entire data frame. However, the AGC hardware has to set the gains of the elements to their maximum value to obtain good sensitivity at all data rates, and thus setting the gains of the elements to lower values, which might permit successful reception of more data frames in the face of interference, is not possible.
Typically known AGC hardware used in receivers of the type described above compensates for interference at the level of analogue to digital converters (ADCs) only. The received signal is scaled such that only part of the available dynamic range of an ADC in the receiver is used. In this way, any interference which appears in addition to the desired signal in the received signal does not saturate the ADC, since there is some “headroom” in the dynamic range of the ADC in which the interference can be accommodated. This is possible because there is a direct relationship between the number of output bits of an ADC and quantisation noise introduced by the ADC.